Multiple sandwich broad band radome



Sept. 26, 1961 s. s. OLEESKY ET AL 3,002,190

MULTIPLE SANDWICH BROAD BAND RADOME Filed April 15, 1955 3 Sheets-Sheet 1 DIRECTION OF EL E C TROMA GNE' TIC a ENERGY PROPAGATION GLASS CLOTH 0.0/0/N. THICK DIELECTRIC CONSIZNT 4.0

POLY/SOCYANATE FOAM 0.072 IN. THICK DIELECTRIC CONSTANT L2 D/REC r/0/v OF 41 4' 7 EL ECTROMAGNE r/c ENE/PG Y PROPA GA T ION GLASS CLOTH 00/0 IN. THICK DIELECTRIC CONSTANT 4-0 POLY/SOCVANA TE FOAM 0.07? IN. THICK DIELECTRIC CONSTANT /.Z

SAMUEL S. OLEESKY, CHARLES E. PEACH, GERALD B. SPEEN L: DONALD H. MC CLURE,

Inventors HUEBNER, BEEHLER,

WORREL & HER ZIG, Attorneys.

TRANSMISSION Sept. 26, 1961 S. S. OLEESKY ET AL MULTIPLE SANDWICH BROAD BAND RADOME Filed April 15, 1955 3 Sheets-Sheet 2 FREQUENCY- KMC SEC.

0 CALCULATED VALUES X MEASURED VALUES SAMUEL S. OLEESKY, CHARLES E. PEACH, GERALD B. SPEEN 8\ DONALD H. MC CLURE,

INVENTORS.

HUEBNER, BEEHLER, WORREL 8 HERZIG,

ATTORNEYS.

p 1961 s. s. OLEESKY ET AL 3,002,190

MULTIPLE SANDWICH BROAD BAND RADOME Filed April 15, 1955 3 Sheets-Sheet 3 REFERENCE AXIS Tz-u T2-l2 T240 TH REFERENCE SAMUEL s. OLEESKY,

CHARLES E. PEACH, GERALD B. SPEEN a DONALD H.MC CLURE,

INVENTORS.

HUEBNER,BEEHLER,

WORREL 8 HERZIG,

ATTORNEYS.

United States Patent 3,002,190 MULTIPLE SANDWICH BROAD BAND RADOME Samuel S. Oleesky, Charles E. Peach, Gerald B. Speen, and Donald H. McClure, Los Angeles County, Calif., assignors, by mesne assignments, to Zenith Plastics Company, Gardcna, Califi, a corporation of Delaware Filed Apr. 15, 1955, Ser. No. 501,629 8 Claims. (Cl. 343-907) The present invention relates to dielectric panels or walls designed to efficiently transmit electromagnetic energy in the microwave spectrum and in particular to such panels or walls constructed of a multiplicity of layers in shapes suitable for radomes and similar housings.

To suit the aerodynamic requirements of high speed flight, radome housings have been designed and developed to take advantage of the dielectric properties of various plastic materials. Several configurations of wall structures for radomes have been developed. These configurations usually consist of from three to five layers of materials of different dielectric constants. Panels or walls having these known configurations are fairly pervious to the transmission of electromagnetic radiation of a narrow range of frequencies and exhibit generally satisfactory transmission efliciencies when the incidence angles of the radiation are not too high and are confined to a very narrow range. The critical performance quality of these known panel configurations with respect to frequencies and incidence angles of electromagnetic radiation has been a serious disadvantage in their use for radome and similar construction Where it is highly desirable that the panels or walls of the structure have broad-band qualities, that is, provide satisfactory electrical performance in connection with the transmission of electro-magnetic waves of wide ranges of frequencies and wide ranges of incidence angles.

Accordingly, it is an important object of the present invention to provide panels or walls for radomes or similar structures or housings characterized in that they elliciently transmit electromagnetic radiation of a wide range of frequencies and a wide range of incidence angles, including very high incidence angles.

Another object is to provide panel or wall configurations comprising a multiplicity of layers of dielectric materials designed to produce panels or walls having high strength qualities and capable of efiiciently transmitting energy in the microwave spectrum over a wide range of frequencies and over a wide range of incidence angles of the impinging energy.

Additional objects will become apparent from the following description.

Broadly stated, the present invention provides a dielectric panel or wall construction comprising a multiplicity of layers, preferably about seven or more. The outermost layer of the construction preferably is of relatively high dielectric constant material, having dielectric constants in the preferred range of about 2.0 to about 40 and especially about 2.7 to about 20. The layer next to the outermost layer preferably is of relatively lower dielectric constant material, having dielectric constants in the preferred range of about 1.0 to about 10 and especially about 1.0 to about 6. Additional inner layers may have dielectric constants of difierent values, preferably falling within a preferred range of about 1.0 to about 20.

A more detailed description of specific embodiments of the invention as applied to radomes is given with reference to the drawings, wherein:

FIGURE 1 is a perspective view showing a panel construction for use as a section of a radome;

FIGURE 2 is a cross-sectional view showing a specific wall configuration comprising six skin layers and five core layers;

FIGURE 3 is a graph showing the transmission of electromagnetic radiation through the wall configuration shown in FIGURE 2 as a function of the frequency of the radiation impinged on the wall at an incidence angle of 30;

FIGURE 4 is a cross-sectional view showing another specific wall configuration comprising four skin layers and three core layers;

FIGURE 5 is a diagram showing the magnitude and phase angle of an electromagnetic wave transmitted through a seven-layer dielectric wall construction of the invention; and

FIGURE 6 is a similar diagram for an eleven-layer dielectric wall construction.

A dielectric panel or wall construction having satisfactory broad-band qualities for use in radomes can be made as shown in FIGURE 2. A first, or outermost skin layer 10 is made of material having a dielectric constant of preferably about 4 and an adjacent, or second, core layer is made of material having a dielectric constant of preferably about 1.2. Succeeding skin layers 12,114, 16, 18 and 20 similarly are made of material having the same dielectric constant as skin layer10, and succeeding core layers 13, 15, 17 and 19 are made of material having the same dielectric constant as core layer 11.

A particular wall configuration was made, as shown in FIGURE 2, to have six skin layers 10, 12, 14, 16, 18 and 20 made of 0.010 inch layers'of glass cloth, such as 1 ply of 181 glass cloth or two plies of 120 glass cloth laminate impregnated with a material such as an alkyd resin or a polyester resin. The dielectric constant of the material in the six skin layers was 4.0. The five core layers 11, 13, 15, 17 and 19 were made of 0.072 inch layers of polyisocyanate foam having a dielectric constant of 1.2. The six skin layers and the five core layers were cemented together with a suitable resin to form a unitary panel or wall structure having a overall thickness of 0.420 inch.

The efiiciency of transmission of both perpendicular and parallel polarized electromagnetic energy of the resulting wall or panel structure was measured for a range of frequencies from 2.0 to 33 kilomegacycles per second over a range of angles of incidence from 0 to 60. By perpendicular polarization it is meant that the electric vector of the incident electromagnetic wave is perpendicular to the plane of incidence and in parallel polarization the electric vector lies in the plane of incidence. The results for perpendicular polarization at an incidence angle of 30 are shown in the graph of FIGURE 3. The graph shows that over percent of the incident elec tromagnetic energy was transmitted by the wall or panel structure over the entire range of frequencies tested and that about percent transmission was achieved over about half of the range.

All of the experimental results are summarized in Table 1 given below:

TABLE 1 Transmission Frequency, Kmc./sec.

Incidence Angle: (9

Transmission Frequency, Krnc./Sec.

Incidence Angle: ("'9 0 98.8 95. 3 96. 3 97.5 98. 6 95. 8 95. 6 95. 4 20. 97. 9 95. 9 95. 6 94. 6 30.-. 96. 9 95. 6 90. 4 96. 3 40 r r 94. 0 95. 4 94. 0 94. 8 50.-- 90. 5 95. 5 89.0 94. 5 55 82. 0 94. 6 83. l 94. 3 60 73. 8 96. 9 77. 6 93. 0

Transmission Frequency, Kme./Sec.

Incidence Angle: C)

Transmission with perpendicular polarized electromagnetic energy. Transmission with parallel polarized electromagnetic energy.

The above table shows that very high transmission of both perpendicular and parallel polarized electromagnetic energy was observed in the panel having the configuration shown in FIGURE 2 over a wide range of frequencies and over a wide range of incidence angles.

A radome panel 21 (FIGURE 1) is made with a wall construction of the type described above by molding the core layers to the desired shape and dimensions. The skin layers are cut to size and cemented between and to the outside surfaces, as the case may be, of the core layer forms to construct the desired configuration. The resulting panel configuration is fixed in a suitable frame to form the finished panel 21 for mounting in a radome either before or after a suitable weather resistant coating is applied to the outer surface of the panel.

The core materials preferably employed in the panel or wall constructions include fabric or fiber laminates and foams such as glass, acrylic, and other fabrics made of synthetic fibers, foams of glass, synthetic plastic foams of polyisocyanates, polystyrene, polyethylene and other synthetic plastics, and foams of cellular hard rubbers, cellular hard resin, and foams of blends of rubbers and resins. The skin materials preferably employed in the wall or panel configurations comprise fabrics of glass fibers, acrylic and other synthetic fibers and fiber laminates or sheets of these fibers.

The thickness of each layer of core material or skin material employed is determined by the dielectric constant of the material and the ranges of frequencies and incidence angles for which the wall structure is to be used. For a given overall thickness of the wall structure, it is desirable to select the dielectric constants, the thicknesses of the successive layers of skins and cores, and the number of such skins and cores so that the amount of reflected electromagnetic energy is reduced to a minimum and the amount of transmitted electromagnetic energy approaches a maximum for a broad range of frequencies and incidence angles of impinging electromagnetic radiation. It has been found that the number and thicknesses of skins and cores of various dielectric constant materials can be determined for a range of frequencies and incidence angles when the skins and cores are chosen so that angles of phase shifts of electromagnetic waves in the successive skins and cores are determined so that the electromagnetic energy transmitted beyond each layer is reinforced to an optimum value over a range of frequencies and incidence angles and is not reduced by cancellation to a relatively much lower value. A method of computing these relationships has been developed which gives satisfactory agreement with test data obtained from computed configurations of wall structures. The method is as follows.

Parameters for single sheets Consider a multi-layer dielectric wall composed of nl lossy, homogeneous, dielectric sheets with radiation of wave length A incident at an angle 0. Consider air as medium 1, the first layer as medium 2, etc., with the subscripts on the dielectric constant (e), the loss tangent (tan 5), and the thickness (d), indicating the layer under consideration.

The parameters for the mth layer are (m=2, 3 n) w/e,,,sin 19+COS 0 for perpendicular polarization.

The equations for the transmission and reflection coefiicients for the mth layer are:

By computing the parameters w, L, and r for Equations 1, 2, and 3a or 3b, and using them in (4) and (5), values of T and R are obtained. a is the phase shift experienced by the wave in passing through the mth layer. L is an arbitrary designation. r is the ratio between reflected and incident energy at the interface between the mth and nth layers. Efliciency is defined as the ratio between the energy leaving a layer and that impinging upon the layer. The transmission co-eflicient is determined by the vector of the wave leaving the layer divided by the vector of the incident wave. Similarly, the reflection co-eflicient is determined by the vector of the wave reflected from the surface divided by the vector of the incident wave falling upon the surface. It should be noted that (4) and (5) hold for waves travelling from right to left as well as from left to right.

it; mm 4) Combining the dielectric sheets where T is the over-all transmission coefficient of the combined sheets for an incident plane wave travelling from left to right.

Ti is the over-all reflection coefficient of the combined sheets for an incident plane wave travelling from left to right.

E is the over-all reflection coefficient of the combined sheets for an incident plane wave travelling from right to left.

T is the transmission coefficient of the dielectric wall p for an incident plane wave travelling from left to right.

T], is the transmission coefficient of the dielectric wall q for an incident plane wave travelling from left to right.

E is the reflection coefficient of the dielectric Wall p for an incident plane wave travelling from left to right.

E2 is the reflection coefiicient of the dielectric wall p for an incident plane wave travelling from right to left.

R is the reflection coefiicient of the dielectric wall q for an incident plane wave travelling from left to right.

R is the reflection coefficient of the dielectric wall q for an incident plane wave travelling from right to left.

Application of equations (1) Two single sheetsmedia 2-3.The transmission and reflection coetficients for each sheet, T T E E, are calculated by use of Equations 4 and 5. These values are then used in Equations 6, 7, and 8 by considering:

33 22 Subscripts indicate direction pq2 R32 (2) Three single sheets-media 2-3-4.-T and E are calculated by use of Equations 4 and 5. These values and those found above are used in 6, 7, and 8 by considering:

(3) n Single sheetswmedia 2-3-4 n.The above procedure is repeated for as many layers as desired, each time using the values of the previous multi-layer as the p-layer for the next calculation.

It may be noted that, in the calculation of sequences of several single sheets, the quantity E is not needed for obtaining T and E of the succeeding multi-layer. Thus, only these latter two quantities need be calculated.

(4) Double symmetrical sandwichmedia 2-3-4-5- 6-7where layers 2, 4, 5, and 7 are identical and layers 3 and 6 are identical.By the above procedure, find the quantities T and E, for the single sandwich under consideration. Combine the two single sandwiches by Equations 6, 7, and 8 by considering:

The transmission curve plotted in FIGURE 3 is typical in its illustration of the close agreement of theoretical, calculated transmission values with actual, measured values for the panel having the configuration shown in FIGURE 2.

FIGURE 5 diagrammatically shows the magnitude and phase angle of an electromagnetic wave transmitted through the first n(n=1, 2, 7, air equals 1) layers of a seven layer broad-band dielectric wall of the invention wherein the first, third, fifth and seventh layers of the wall had a dielectric constant of 4.0; the second, fourth and sixth layers had a dielectric constant of 1.2 and were 0.250 inch thick; the first and seventh layers were 0.020 inch thick; and the third and fifth layers were 0.010 inch thick. The diagram shows a reinforcement of the magnitude of the electromagnetic wave as it passes through the wall. It will be seen that vector T is much shorter than vector T which is the vector through the entire wall structure.

FIGURE 6 is a similar diagram for an eleven layer broad-band dielectric wall of the invention wherein the dielectric constant for the first, third, fifth, seventh, ninth and eleventh layers was 4.0 and the thickness was 0.010 inch; and the dielectric constant for the second, fourth, sixth, eighth and tenth layers was 1.2 and the thickness was 0.072 inch. This diagram shows the rotation of the transmission vector through approximately equal angles for equal layers and shows some reinforcement of the magnitude of the electromagnetic wave.

Instead of the six skin and five core layer wall structure discussed above in connection with FIGURE 2, a structure having fewer layers, such as that shown in FIGURE 4, for example, can be used. Namely, a Wall structure having four skin and three core layers of 4.0 and 1.2 dielectric constant, respectively, has been found to give satisfactory broad band performance characteristics. It is not necessary in every case that both the outermost and the innermost layers be skins of high dielectric constant material to have wall structures of broadband characteristics. The innermost layer can be a core layer of relative low dielectric constant material, for example. Other examples of specific wall or panel configurations employing skin layers of material having a dielectric constant of 4.0 and core layers of dielectric constant 1.2 have been made with the following configurations and transmission characteristics.

EXAMPLE 1 TABLE 2 [Angle of incidence 30] Transmission Frequency, mc./sec.

* Perpendicular polarized ener Parallel polarized energy. gy

7 TABLE 3 [Angle of incidence 60] Frequency, mc./sec. Transmission 1* See footnote, Table 2.

EXAMPLE 2 The first, third, fifth, seventh, ninth and eleventh layers were skins. The first and eleventh layers were 0.020 inch thick and the third, fifth and seventh and ninth layers were 0.010 inch thick. The second, fourth, sixth, eighth and tenth layers were cores 0.072 inch thick and the overall thickness was 0.440 inch. Table 4 contains a summary of transmission values calculated at an incidence angle of 30".

TABLE 4 Frequency, mc./sec. Transmission 33,000 69.87 24,000 70.39 16,000 81.78 12,000 39.83 9,375 98.13

* See footnote, Table 2.

EXAMPLE 3 The first, fifth, seventh and eleventh layers were skins of 0.010 inch thickness and the third and ninth layers were skins of 0.020 inch thickness. The second, fourth, sixth, eighth and tenth layers were cores of 0.072 inch thickness and the overall thickness was 0.440 inch. Table 5 contains a summary of the transmission data obtained at an angle of incidence of 30.

TABLE 5 Frequency, mc./sec. Transmission 33,000 91.91 24,000 94.11 16,000 80.59 12,000 94.17 9,375 98.53 7,000 90.67 4,000 90.97

See footnote, Table 2.

EXAMPLE 4 The first and eleventh layers were skins of 0.020 inch thickness and the third, fifth, seventh and ninth layers were skins 0.010 inch thick. The second, fourth, eighth and tenth layers were cores 0.060 inch thick and the sixth layer was a 0.120 inch thick core. The overall thickness was 0.440 inch. See Table 6 for transmission data obtained at an angle of incidence of 50.

TABLE 6 Frequency, mc./sec. Transmission 33,000 85.49 24,000 91.32 16,000 61.36 12,000 90.12 9,375 94.89 7,000 84.66 4,000 84.60

* See footnote, Table 2.

EXAMPLE 5 The first and thirteenth layers were skins 0.020 inch thick and the third, fifth, seventh, ninth and eleventh layers were skins 0.010 inch thick. The second, fourth,

8 sixth, eighth, tenth and twelfth layers were cores 0.060 inch thick, the overall thickness being 0.450 inch. Table 7 contains a summary of transmission values obtained at an incidence angle of 50.

TABLE 7 Frequency, mc./sec. Transmission 33,000 85.92 24,000 74.17 '16,000 68.64 12,000 90.67 9,375 94.30 7,000 82.81 4,000 82.37

' See footnote, Table 2.

EXAMPLE 6 The wall structure was the same as in Example 5 except that the seventh layer was 0.020 inch thick instead of 0.010 inch thick and the overall thickness was 0.460

The wall structure was the same as in Example 6 except that the second, fourth, sixth, eighth, tenth and twelfth layers were 0.072 inch thick instead of 0.060 inch thick and the overall thickness was 0.532 inch instead of 0.460 inch. Transmission values were calculated at incidence angles of 30 and 50. Tables 9 and 10 summarize the results.

TABLE 9 [Angle of incidence 30] Frequency, mc./ sec. Transmission See footnote, Table 2.

TABLE 10 [Angle of incidence 50] Frequency, mc./sec. Transmission See footnote, Table 2.

It will be observed that all of the wall structures exemplified above exhibit highv transmission of electromagnetic energy over a wide range of frequencies.

Although most of the wall or panel structures discussed above had a total number of layers of skins plus cores equal to about seven, eleven, or thirteen layers, it is to be understood that any total number of layers can be used, limited only by practical feasibility. A total of twenty to thirty, or more layers is believed practical or feasible in actual practice.

The foregoing description is primarily for explanatory purposes, and is given chiefly to illustrate certain specific embodiments of the invention. It is understood that many variations in the structure, configuration and details of the wall or panel structures described above will occur to one skilled in the art. Accordingly, it is understood that such changes and modifications in the structure, configuration and details of the specific embodiments of the invention illustrated and described above may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

'1. A dielectric wall for the transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising an outermost skin layer of material having a dielectric constant of from 3.5 to 4.5 and a thickness of from 0.01 to 0.02 inch, an inwardly adjacent core layer of material having a dielectric constant of from 1.05 to 1.4 and a thickness of 0.06 to 0.075 inch, and inwardly alternating skin and core layers of said materials having dielectric constants and thicknesses substantially the same as said first skin and core layers, respectively, there being at least four of said skin layers and at least three of said core layers, and thicknesses of 0.01 to about 0.02 inch and 0.06 to 0.075 inch, respectively.

2. A dielectric wall according to claim 1, wherein there are six skin layers and five core layers.

3. A dielectric wall according to claim 1, wherein the skin layers comprise glass fabric and the core layers comprise a polyisocyanate foam.

4. A dielectric wall for the transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising spaced skin layers substantially 0.02 inch thick, two core layers adjacent and between the skin layers substantially 0.06 inch thick, two skin layers adjacent and between said core layers 0.01 inch thick, two core layers adjacent and between the last-mentioned skin layers 0.06 inch thick, two skin layers adjacent and between the last-mentioned core layers 0.01 inch thick, and a middle core layer 0.12 inch thick, the skin layers containing glass fabric and having a dielectric constant of 4 and the core layers containing a polyisocyanate foam and having a dielectric constant of 1.2.

5. A dielectric wall for the transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising seven skin layers alternating with six core layers, the outermost and innermost skin layers being 0.02 inch thick and the five remaining skin layers being 0.01 inch thick, each of the core layers being 0.06 inch thick, the skin layers containing glass fabric and having a dielectric constant of 3.5 to 4.5 and the core layers containing a polyisocyanate foam and having a dielectric constant of from 1.05 to 1.4.

'6. A dielectric wall for the transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising spaced skin layers defining opposed outer faces thereof and having a dielectric constant of from 3.5 to 4.5, a core layer inwardly adjacent each skin layer, .072 inch thick and having a dielectric constant of from 1.05 to 1.4, and additional inwardly alternating skin and core layers, there being at least four skin layers and at least three core layers, said outer skin layers being .02 inch thick, the remaining skin layers being .01 inch thickness.

7. A dielectric wall for the transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising spaced skin layers defining opposed outer faces thereof and having a dielectric constant of from 3.5 to 4.5, a core layer inwardly adjacent each skin layer, .072 inch thick and having a dielectric constant of from 1.05 to 1.4, and additional inwardly alternating skin and core layers, there being at least four skin layers and at least three core layers, said outer skin layers being .01 inch thick, the next innermost skin layer being .02 inch thick, and the remaining skin layers being .01 inch thick.

8. A dielectric wall positioned with respect to a source of electromagnetic energy for transmission of a major proportion of electromagnetic energy transversely therethrough over a broad band of frequencies comprising an outermost skin layer of material having a dielectric constant in the range of 2.0 to 40, an inwardly adjacent core layer of material having a lower dielectric constant in the range of 1.0 to 10, and inwardly alternating skin and core layers of said materials having dielectric constants in said higher and lower ranges, respectively, there being at least four of said skin layers and at least three of said core layers.

References Cited in the file of this patent UNITED STATES PATENTS 2,511,610 Wheeler June 13, 1950 2,577,463 Hansell Dec. 4, 1951 2,617,934 McMillan et al. Nov. 11, 1952 2,639,248 Overholt May 19, 1953 2,642,920 Simon et a1 June 23, 1953 2,659,884 McMillan et a1. Nov. 17, 1953 OTHER REFERENCES Cady: Radar Scanners and Radomes, vol. 26, Radiation Lab. Series, pp. 277-8 and 390-393.

Report No. NADO-EL-5293; August 6, 1952; Reflection and Transmission of Electromagnetic Waves by Multilayer Plane Dielectric Sheets at Arbitrary Incideuce; reported by Samuel Wolin; pp. 5 to 10.

Report No. NADC-EL-SZ 188; October 22, 1953; Phase Report Tables of Transmission and Reflection Coefiicients of Lossy, Symmetrical Dielectric Radome Sandwiches. Reported by Samuel Wolin; pp. xii, xxiii to xxv. 

1. A DIELECTRIC WALL FOR THE TRANSMISSION OF A MAJOR PROPORTION OF ELECTROMAGNETIC ENERGY TRANSVERSELY THERETHROUGH OVER A BROAD BAND OF FREQUENCIES COMPRISING AN OUTERMOST SKIN LAYER OF MATERIAL HAVING A DIELECTRIC CONSTANT OF FROM 3.5 TO 4.5 AND A THICKNESS OF FROM 0.01 TO 0.02 INCH, AN INWARDLY ADJACENT CORE LAYER OF MATERIAL HAVING A DIELECTRIC CONSTANT OF FROM 1.05 TO 1.4 AND A 